Fix mute-button in mediaplayer

Fix for attributeError: 'EmbeddedPlayerPanel' object has no attribute 'oldvolume'
Traceback (most recent call last):
  File "/home/rob/git/tribler/Tribler/Main/vwxGUI/EmbeddedPlayer.py", line 205, in MuteClicked
    self.volume = self.oldvolume
AttributeError: 'EmbeddedPlayerPanel' object has no attribute 'oldvolume'